./! 
 
 
 
 WOOD BENDING IN SHIP BUILDING 
 
 April 1943 
 
 U. S. DEPT. OF AGRICULi 
 LI BRARY 
 
 ATLANTA BRANCH 
 
 MAY 6 mt 
 ATLANTA, Gc.</ftGIA 
 
 ■ »■ ^ » ■ ■> ^- i 
 
 No. R143C 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST SERVICE 
 
 FOREST PRODUCTS LABORATORY 
 
 Madison, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
' 
 
 Ei C, PEC . 
 
 A lar roportion < 
 of ■ • ce only a red 
 
 cr etches or cut from stumps, 
 ber of rectar ; sre shape, c 
 
 ■ ous contc it re ! • 
 
 -.atcriai and do no1 , becaut 
 
 miform i1 th along its 1 g a 
 
 er -o con desired cur ' of 
 
 the str " al 
 
 crooks or less thaj 
 
 mbers. TV 3f wood to curved shapes is \ 
 
 racticed. I - ' ' " aiowledg * s of 
 
 fun' • t ■ . Dhe larg 
 
 that current practic 
 often fall short. 
 
 - building: of rood 3] L] , there are n 
 satisfactori ending. 3 tally tr 
 
 smaller 9 whose r ' 
 
 to bend. Sc bending can as the 
 
 place; for example, planlri -ibs at tl a of 1 Llge. 
 
 is sometimes for"-- I or ' ' 
 
 • e bpnds ar.i twists are liable tc vere it is 
 
 tc the planks or soak their, in hot v I 
 
 . •- • ads of 1 -ibs ar iot severe I 
 
 ■ - - and pu] 
 
 cr ribbands. -As the frame toi - • s each stringor or r is 
 
 in place. One dis< bage of tr the tei 
 
 strait: ^ r cooling. 3 rids mus1 a f c 
 
 able or the re fas 1 
 
 pla . 
 
 Lon of bent wood pari 
 : (I) Lection of stock; of stock; (3 - o of 
 
 : 4) sofl ~ of th" 7/ood; ' operation; 
 
 . 
 
 -od of all s] 
 
 o 'om 1 
 
 les ca: led. Oak, elm, ash, : . and 
 
 for prod t s+o- , 
 
 • . . 
 
been used in ship Truilding and --.re preferred for bendin . Leces of 
 
 wood from a species ordinarily used for bending are suitable.. 7r.< wood 
 be straight-grained; the deviation in spiral or cross-grain should not 
 exceed 1 inch in 15 inches. This applies to * a In of thi | - as a 
 whole and also to irregularities in the vicinity of knots and burls. 1' 
 wood must be free from decay, Wood of high density is desirable because of 
 its superior strength properties but with proper technique 7/ocd of hi 
 density or with a particular rate of growth is net essential to success in 
 bending. Wood of high density has a high percentage of summerwood, but it 
 is difficult to determine this percentage in hard-roods. An indirect method 
 is to noi number and size of the pores. Defects such as knots, worm 
 holes, shakes, and surface checks are detrimental, since they tend to cause 
 failures, but small sound knots near the ends, moderate surface checking . 
 and scattered worm holes are permissible. A piecp containing ring shakes 
 or the heart center of the trep is net suitable for bending. 
 
 Seasoning of Stock 
 
 There is considerable difference of opinion regarding mois- 
 ture content of the stock at the time of bending. Bending is performed on 
 wood whose moisture content ranges from the green condition down to 12 
 percent. Green wool i '. sxceptionally high moisture content is not suit- 
 able for severe bends because hydrostatic pressure may develop cr. the side 
 in contact with the form and cause compression failures. From a theoretical 
 standpoint the optimum moisture content is probably in the. vicinity of the 
 fiber-saturation point, from 2^ to J>G percent moisture content. 
 
 Since wood for bending cannot I pt 'long in the green condition 
 
 bhout the development of stain and decay (except during cold t< a1 er) 
 most stock must undergo some seasoning before bending. Material c 
 warm weather must be protected from sun and rain and piled on stickers, 
 whil^ waiting shipment. On arrival at the ship building plant the stock 
 should be piled on stickers either in a shed or in an air seasoning pil.e 
 protected from sun and precipita/ During cold weather less stringent pre- 
 
 are required because stain and decay organisr r relatively in- 
 active. It is good practice to apply a moisture r< sistant coating to the 
 
 ■ of the pieces (immediately after they are cut) to reduce the amo i 
 of end ing and end splitting. 
 
 Tr-par-^tior. of Bending Blanks 
 
 Pieces for any particular bend should be trimmed to uniform 1 
 the ends square with the length. 1 e d shoul :ept to "- mini- 
 fflUii enough allowance for shrinkage. To prevent side buckling t 
 
 width should be greater than the d I , Tieces are sometimes bent in 
 
 multiple--, of desired widths and sawed after bending. Surfacing the side 
 
 next to the form and the two adjacent faces will reduce the danger of com- 
 p ress i failure , 
 
 liimeo. To. RIH30 
 
■"of- 
 
 ' ' • ■ . 
 - than in c 
 
 nt more successful - 00 & 
 
 ox, in 1 res< nee of - 
 
 or *:o submerge the wood in boilir 
 
 1 hour :r.ess. Chemicals hav 
 
 plastic, but ood for 
 
 been thoroi hly Lnvi stigat' 
 
 The E endin • Cr pra.ti 
 
 on 
 
 softenii •. given sever be 
 
 : in a bendin 
 
 ii - rids can 
 
 -ore bend- or 
 
 Led on 1 I nsion side, whicl: i - -side c: 
 is designed to prohibit tensic 
 
 designed end bloc adjusting 1 
 
 - ically all of the • - 
 
 forced to take place as upset, i - ave si" 
 
 a minimum at the convex side. The end bloc mist be provide;-. L1 
 for relieving the end pressure as bending progresses. The end pr 
 be gradual,. iced as bending progress 
 
 • betwee I ids of the 31 rid blocks of 
 
 strap, blocks of rood that receive thi 
 •ush ?.s bending progresses. It is necessary to desi . . 
 end blocks will not overturn. - so that \ 
 not r itted to acquire a reverse bend in 1 of t 
 
 (figs. 1, 2, & 3). As the bend progresses the are: 
 tne form has a tendency to bac y. "o counters 
 
 re to hold the portion aln i ainst the for. -ovided, 
 
 •ience and observations have s> 1 
 - st essential feature of the whole operation i 
 tension and end pressure during the formatioi of th< bend. 
 reversed lever principle as illustrated in figui 
 ffectiv ays of controlling end pressure and str 
 
 odied in the bending aonaral .;■ 
 
 ing and usir -us of this typi oi i 
 
 ■ II of hi oss of | 
 
 elimii "• • -■-: " boal she be able to make ■ that 
 
 but rnose dimensions end otl Lis of 
 construction ar pt to the bent parts being prod 
 
 Shar e 
 
 r to make 1 
 
 :s is cor 
 or mo] - e allowed to remai 
 
has seasoned enough to hold its share. Sometimes tension failures, on the 
 outer side of the curve, take place during this drying process. To pr?- 
 this a minor strap is sometimes applied to the piece before bendin : . 
 bending, a chain is attached to the ^nds of this strap, which assists in 
 holding the shape of the piece and in retaining sufficient end pressure to 
 prevent tension failure during seasoning. 
 
 Sometimes bent parts check severely during cooling. Such checking 
 is likely to be severe in stock that is bent while green or partially 
 seasoned. Heat stored in the bending blank during the softening process 
 causes very rapid evaporation. When the piece is cooled in the air after 
 bending shrinkage occurs in surface layers before the interior of the 
 piece has lost sufficient moisture to cause it to shrink. Cooling under 
 water at normal temperature prevents this rapid evaporation and permits 
 the subsequent drying to take place more slowly. 
 
 i.'ineo '„ T o . HIU3O 
 
• LUeruMtic ak»tch of a b«n41n< itrap ha>Tln« l&*r 
 
 •hort • nd block*. A, B*for« proaaura; (P) la tpplladi 8, 
 
 aft«r praaaura la appllod. 
 
 lota how oad block* tip owr and alldo off. 
 
 b*od ut»r and • 
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 UNIVERSITY OF FLORIDA 
 
 ill i 
 
 3 1262 08924 5558